Ink holdout is resistance to the penetration of ink. Coated papers tend to have good ink holdout. The ink pigments sit on the surface of the coating, and are not absorbed into the spaces between the paper fibers. This minimizes dot spread and results in a sharp image.
Uncoated papers tend to absorb ink into the sheet, but printers can compensate for this and still produce a very bright, sharp image on uncoated paper. See also coated paper, dot compensation, and ink absorption.